Evasive Maneuvers

Friends, readers, countrymen, lend me your ears. Eyes. Whatever. I need your help with a delicate problem. It seems I have accidentally befriended the craziest person at my work. This is a woman twice my age who apparently wants to be my best friend, which I would normally not have a problem with (who wouldn't want to be my friend?) except for the nagging fact that she is completely batshit fucking crazy. Before I was aware of this little tidbit, I agreed to go walking at lunch sometimes with her. This happened through a rather boring course of events which I will spare you (see how kind I am?). Shortly after these plans were made, things began to go awry. Upon finding out that my Easter plans included a hike up the mountain (which never did happen, thanks to my damn allergies), she expressed a burning desire to go for walks on the beach on the weekend with me. I quickly deterred this plan by saying that I never know what my weekend plans are until the day before, which is largely true. This weekend is an exception, but more on that after it's all over and I've gotten some sleep.
Moving on. After I told her that weekends aren't good for me to plan, she tells me that she understands, since she is really busy on weekends too. Huh? Then why are you trying to plan things with me?
But wait, there's more! Earlier this week, I asked her a work-y question. She responded briefly and then expounded on how much she was looking forward to our walk (which was today). I mentioned that Pam might like to join us, since she works next door and we usually have lunch together. Apparently, this did not sit well with the crazy one, who them sent me six emails in less than three minutes explaining that she preferred one on one walking, and that she used to work next door and "wanted to pass on getting up-to-date on them", because clearly that's all Pam would talk about. I have to work with her still, so I just decided to go with it and have it be just me and her, creepy as she was getting. She was extremely effusive in her thanks, which just bumped up the Creep-O-Meter. The phrase "one-on-one the first time" was used.
So The Day arrived today, and it turns out that she forgot to bring her lunch, so she wants to walk a bit and then go get some food. How convenient. At first she was angling for this atrocious Hawaiian barbecue place that the smell of makes me nauseous, but I suggested sushi and she agreed. Things are going to get a bit play-by-play here because it just happened. We went to the park and walked for a bit, during which time she complained about people at work. Then we headed over to the sushi place, where she was all kinds of particular about where we sat. I don't want to keep writing "and she talked about people at work", so just imagine that during all of the events I am describing, she is doing just that. Constantly. Good. We finally got our food, at which point she gushed for a bit about how wonderful sushi is. Which it is and all, but for fuck's sake, she got a Cali roll and a teriyaki bowl. I had a Spicy Tuna roll and a teriyaki bowl. We ate for a bit and she talked the entire time with food in her mouth. I cannot stand that.
She asked my what I had, and (brace yourself, this is gross) poked my Spicy Tuna roll with her fork that she had been eating off of! I didn't know what to do! I barely know this woman, and she's getting her germs all over my food. Shudder. I don't want to think about it anymore. I then had to explain to the Food-Toucher what was in my Spicy Tuna roll, since it isn't, y'know, self-explanatory at all. Then she says, and this is gold, "Oh I'm an expert on California rolls, but anything else I have no idea." Am I to take that it is hard to know all there is to know about California rolls? Seriously?
My question to you, dearest readers, is how I can politely deter her crazy advances without mucking up things at work. Keep in mind that she's crazy and I don't want to wake up to my cat's severed head or something.